purposefulness: speakers are motivated by a communicative goal (such as getting information, making a request, giving instructions) and not simply by the need to display the correct use of language for its own sake;
reciprocity: to achieve this purpose, speakers need to interact, and there is as much need to listen as to speak;
negotiation: following from the above, they may need to check and repair the communication in order to be understood by each other;
synchronicity: the exchange – especially if it is spoken – usually takes place in real time;
unpredictability: neither the process, nor the outcome, nor the language used in the exchange, is entirely predictable;
heterogeneity: participants can use any communicative means at their disposal; in other words, they are not restricted to the use of a pre-specified grammar item.